This invention relates to the art of casting composite articles. It is particularly applicable to casting improved lightweight pistons and will be described with particular reference thereto. It will be appreciated, however, that the invention also finds application in casting other composite articles, particularly those which include wear resistant inserts.
Aluminum alloys and other lightweight materials are advantageously used in the manufacture of pistons. One method of casting aluminum and other alloys is known as squeeze casting. In squeeze casting, a female die cavity is fashioned in the shape to be cast. The die is open at the top to allow a molten alloy to be poured into the die cavity. The die is closed by a top punch which is inserted into the upper opening of the die cavity. The punch exerts a pressure on the molten metal which cntinuously forces the alloy against the walls of the die cavity as it solidifies. The top punch enters the die cavity further with shrinkage. An article cast by the squeeze casting technique has good conformity to the die cavity surface, has a fine microstructure, and relatively little or no porosity.
Aluminum and many other lightweight alloys tend to wear quickly. This lack of wear resistance makes aluminum and other lightweight alloy pistons undesirable for heavy-duty engines such as are found in large trucks, large farm vehicles, and off the highway equipment. To improve the wear resistance of aluminum and other lightweight pistons, it has been suggested that a ring of wear resisted material be inserted around the piston. Examples of such composite piston structures are illustrated in U.S. Pat. No. 4,008,051, issued Feb. 15, 1977 to T. M. Cadle, U.S. Pat. No. 3,533,329, issued Oct. 13, 1970 to E. Galli, U.S. Pat. No. 2,956,846, issued Oct. 18, 1960 to W. E. McCullough, and U.S. Pat. No. 2,550,879, issued May 1, 1951 to C. E. Stevens, Jr.
These composite pistons are cast in a permanent mold which has runners, gates, or risers for introducing molten metal into the mold cavity at the proper places and rates. To allow for shrinkage, as much as 40% extra alloy is poured into the mold, runners, gates, and risers. When the metal solidifies, the two or more parts of the permanent mold are opened and the piston is removed. Various machining steps are needed to cut off the excess metal, adjust the piston dimensions for shrinkage, and prepare it for precise finish machining. The piston tends to be weakened by porosity and a coarse microstructure, both of which are attributable to shrinkage from the die walls during solidification.
The wear resistant insert ring, commonly an iron alloy, and the aluminum piston body have different physical properties. For example, the iron alloy's specific gravity is generally 2 to 3 times that of the aluminum alloy, the iron alloy's thermal expansion coefficient is generally 1 to 11/2 times that of the aluminum alloy, the iron alloy's thermal conductivity is generally less than half that of the aluminum alloy. These different physical properties cause residual stresses in the composite pistons.
To resist these stresses, it is essential that a strong bond be formed between the wear resistant ring and the aluminum alloy. In its normal life cycle, a composite piston is subject to the shocks and vibrations of innumerable firing cycles as well as numerous heating and cooling cycles from starting and stopping the engine. Even a minute crack or bonding failure between the wear resistant ring and the aluminum alloy can propagate quickly under these adverse conditions. The propagation of cracks is accelerated by the formation of a brittle aluminum-iron alloy or the formation of oxides at the interface. The propagation of cracks can cause pieces of pistons to break loose resulting in catastrophic engine damage.
Although composite articles have been squeeze cast or molded in the past, note U.S. Pat. No. 3,792,726, issued Feb. 19, 1974 to Sakai et al., U.S. Pat. No. 2,157,453, issued May 9, 1939 to Jaegar, U.S. Pat. No. 1,950,356, issued Mar. 6, 1934 to DeBats, and Japanese Patent 9557, issued July 4, 1961 to Iwamura et al. (Chemical Abstracts 14862(h), 1962), squeeze casting of pistons has not, heretofore, been successful. This may be attributable to the difficulty in achieving adequate bonding, particularly to the lower side of the wear resistant ring. This may also be attributable to the additional internal stresses from flexing of the wear resistant ring under the forces from the top punch.
One of the principal problems in composite piston casting techniques is achieving a strong, fracture resistant bond between the wear resistant ring and the aluminum or other lightweight alloy.
Yet another problem with casting composite pistons has been the number of machining steps and other labor processes required to finish the cast product.
The present invention contemplates a new and improved method and apparatus for casting composite articles, particularly pistons, which overcomes the above-referenced problems and others. Yet it provides a composite piston in which the unlike metal alloys are strongly bonded, which is crack and fatigue resistant, and which is finished with fewer machining steps.